In optical communication systems, photodetectors based on compound semiconductors, such as InGaAsP, etc. are widely used. However, a photodetector comprising a light absorption layer of a Si/SiGe superlattice layer is preferred for the chemical compound based photodetector in view of its consistency with Si processing. A Si-based opt-electronic integrated circuit, on which Si-transistors and Si-based photodetectors are simultaneously integrated, is now being developed.
The structure of the photodiodes is roughly classified in two types, one is a surface incidence type and the other is a waveguide type. In the photodetector of the surface incidence type, the direction of the incident light is perpendicular to the surface of the substrate, while in the photodetector of the waveguide type, the direction of the incident light is parallel with the surface of the substrate. In the photodetectors of both types, photo-sensitivities can be increased by increasing the depth of the light absorption layer. In the photodetector of the surface incidence type, since the light incident path of the light can be increased, as the thickness of the light absorption layer is increased, the light reaches a deeper portion of the light absorption layer and the absorbed light increases, hence, the photo-sensitivity of the photodiode increases. On the other hand, in the photodetector of the waveguide type, received light power of the photodetector increases, as its receiving cross-sectional area for the incident light, which is emitted from the output end of an optical fiber, becomes larger, and subsequently the photo-sensitivity of the photodetector increases.
The greater part of the photodetectors used at present are of the surface incidence type. The common feature of the photodetectors of surface incidence type is that a light absorption layer is formed in a semiconductor epitaxial layer, and an end region of the light absorption layer is communicated with the cathode electrode of the photodetector via a part of the epitaxial layer, which runs in parallel with the side wall of the light absorption layer. If the depth of the light absorption layer is increased in order to increase a photo-sensitivity, a distance from the cathode electrode to the end region of the light absorption layer inevitably increases. Since the conductivity of the epitaxial layer is low in most cases for several reasons mentioned later, a resistance between the cathode electrode and the end region of the light absorption layer increases in the aforementioned case, which is unfavorable from the aspect of quick photoresponse of the photodetector. Thus, there is a trade-off between response and sensitivity. Some efforts have been made for decreasing the aforementioned cathode circuit resistance, but the structure of the photodetector becomes complicated and the time required for fabrication process increases.